1. Technical Field
The present invention relates to a method of fabricating a semiconductor device in which plural bipolar transistors having different DC current gains (hfe) are hybridized (consolidated).
2. Related Art
Conventionally, technologies have been proposed which, in a case where bipolar transistors having various emitter areas are placed on the same semiconductor substrate in logical LSIs and analog ICs, suppress variations in the DC current gains (hfe) in an IC in which bipolar transistors having emitter regions of different sizes are hybridized (e.g., see Japanese Patent Application Laid-Open (JP-A) No. 11-121460).
On the other hand, there is also a case where two bipolar transistors having different DC current gains (hfe) are hybridized on the same semiconductor substrate. In this case, there is a method of hybridizing the two bipolar transistors having different DC current gains (hfe) on the same semiconductor substrate by changing the concentration of an impurity diffusion layer that becomes emitter regions, base regions, and collector regions.
Here, the depths at which embedded electrodes reach the emitter regions can be varied by changing the contact depths (the depths of contact holes) because the emitter regions are shallow and their impurity diffusion layer concentration gradient is large, so the effective impurity diffusion layer concentration changes, and the DC current gains (hfe) can be changed. For this reason, in a case where this property is utilized to hybridize two bipolar transistors having different DC current gains (hfe) on the same semiconductor substrate, the hybridization can also be realized by changing the contact depth of each bipolar transistor. Of course, the hybridization can be realized by changing the contact depth also in another region (the base region, the collector region).
However, in order to change the concentration of the impurity diffusion layer that becomes the emitter regions, the base regions, and the collector regions, it is necessary to divide implantation, and it is necessary to add implantation steps depending on the number of impurity diffusion layers of the concentrations intended.
Further, also in order to change the contact depths of the emitter regions, it is necessary to add a photolithography/etching step depending on the number of the contact depths intended, and the constraints are large particularly in a process where only the formation of contact holes with diameters of a single size is allowed.